1. Field of the Invention
This invention relates to a synchronization method proposed for the terminal-to-terminal direct communication (simplified as the direct communication hereinafter) under the Time Division Duplex mode (simplified as TDD hereinafter) in a Mobile Communication System.
2. Description of the Prior Art
Synchronization is a key technology in the TDMA communication system. Similar to other TDMA systems, a TDD system needs accurate synchronization between terminals and a base station.
Due to the mobility of the terminals in the mobile communication system, the distances between the terminals and the base station are variant, so the time delay of the signals transmitted from the terminals to the base station via wireless channel is thus variant. Thus, the synchronization issue of TDMA mobile communication system becomes more complex and important.
In the existing TDD system, considering the radio propagation delay, each terminal transmits signal in advance with certain timing based on its distance from the base station in order to compensate for the propagation delay, which enables signals from different terminals arrive at the base station synchronously after different propagation delays. The receiving timing of the base station is a reference for transmission timing of other terminals. The base station sends a synchronization instruction to the terminal, and the terminal adjusts its transmission timing timely according to the synchronization instruction, which makes the uplink signals of all the terminals realize complete synchronization at the base station.
At present, the standards specified by 3rd Generation Partnership Project (3GPP) still didn't concern with the direct communication. All synchronizations in the existing standards are synchronization between the terminals and the base station. The differences between the direct communication and the normal communication consist in that the direct communication is realized directly between terminals without participation of the base station, and it is a direct signal transmitting/receiving between the terminals, wherein the sender and the receiver are both terminals and participation of the base station is not needed during the communication process. In this case, the synchronization methods in the former standards are not applicable to the synchronization of the direct communication. Thus we need to specify a new synchronization method for the direct communication.